1. Field of the Invention
This invention relates to a crystallizing method suitable for use in a display device, such as a liquid crystal or organic electroluminescence (EL), a thin-film transistor manufacturing method, a thin-film transistor, and a display device.
2. Description of the Related Art
The driving circuit for a display device, such as a liquid-crystal display device, has been fabricated by using an amorphous semiconductor film formed on a glass substrate. With the expansion of the IT services market, information handled in this field has been digitized and speeded up and therefore display devices have been required to provide higher picture quality. One of the means for meeting the requirement is such that, for example, switching transistors for switching the corresponding pixels are fabricated by using a crystallized semiconductor, thereby making the switching speed faster, which enables higher picture quality.
One known means for crystallizing an amorphous silicon layer formed on a glass substrate is the excimer laser annealing method (ELA method). The grain size of a polycrystalline silicon obtained by the ELA method is about 0.1 μm. When a thin-film transistor (TFT) is formed in the crystallized region, a large number of grain boundaries are included in the channel region of a single thin-film transistor, which gives an electron field-effect mobility of 200 cm2/Vs. This mobility is much inferior to that of a MOS transistor fabricated by using a silicon single crystal.
The inventors of this invention have developed the industrial technique for forming as large a crystal grain as enables a channel region of at least one thin-film transistor to be formed by irradiating an amorphous silicon layer with a laser beam. Forming a TFT in a single crystal grain has no adverse effect from the grain boundaries on transistor characteristics which differ from the conventional transistor characteristics where the grain boundaries are formed in the channel region. In addition, the TFT characteristic is improved remarkably and a functional element, such as a processor, a memory, or a sensor, can be formed. As such a crystallizing method, the inventors have proposed the crystallizing method described in, for example, W. Yeh and M. Matsumura, Jpn. Appl. Phys. Vol. 41 (2002)1909 and M. Hiramatsu, et al., Extended Abstracts (The 63st Autumn Meeting, 2002); The Japan Society of Applied Physics No. 2, P779, 26a-G-2.
The document by W. Yeh and M. Matsumura has described a method of irradiating a laser beam phase-modulated at a fluence of 0.8 J/cm2 to an amorphous silicon film via an SiON/SiO2 cap layer or an SiO2 cap layer, causing a Si grain to grow laterally in parallel with the film, which crystallizes the amorphous silicon film.
The document by M. Hiramatsu, et al., has described a method of irradiating a laser beam homogenized and intensity-modulated to an amorphous silicon film via an SiO2 cap layer, while the substrate is being heated, which causes the amorphous film to grow in crystal laterally.
In the method of the document by W. Yeh and M. Matsumura, a Si grain whose size is equal to or larger than 10 μm can be obtained. Since very small grains with small sizes appear near the large grains whose sizes have become larger, it is expected that the grains with large grain size are formed in the same size so as to be relatively uniform (or close) as an overall film structure. The SiON cap film is capable of changing the absorption spectrum by changing the ratio of oxygen atoms to nitrogen atoms in the film. However, even a film whose optical bandgap is the smallest (SiNx without oxygen) has about 5 eV (This bandgap energy corresponds to nearly 240 nm in wavelength). Thus, it can be used for KrF laser whose wavelength is 248 nm among excimer lasers, but is difficult to use for XeCl laser now often used in mass production whose wavelength is 308 nm, because it becomes transparent. This is a problem.
Furthermore, in the methods in the document by W. Yeh and M. Matsumura and the document by M. Hiramatsu, et al., the substrate has to be heated to a high-temperature region to make crystal grains larger in its size, preventing the requirement for low-temperature treatment from being met, which is a problem. For example, a conventional crystallizing apparatus 100 shown in FIG. 11 is a unit which irradiates a pulse laser beam 105 emitted from a KrF excimer laser unit 104 to a substrate 103 to be crystallized which is heated to a high-temperature region by a heater 102 built in a table 101, thereby crystallizing the substrate. The pulse laser beam 105 is a laser beam passed through an optical system composed of a concave lens 106, a convex lens 107, and a phase shifter 108. A power supply 110 controlled by a controller 109 supplies electricity to the heater 102, which has the capability of heating the substrate 103 to a temperature range from 300 to 750 degrees.
Since the substrate heating temperature may exceed, for example, 500 degrees, general-purpose glass (e.g., soda glass) or plastic is liable to deteriorate or deform due to heating. Therefore, to use these materials as substrates for a liquid-crystal display (LCD), low-temperature treatment is an indispensable condition. Large-screen LCDs have been strongly required to be lighter. Therefore, their substrates tend to be made thinner and therefore are liable to deform due to heating, which makes low-temperature treatment an indispensable condition to secure the flatness of the thin substrates.
Furthermore, since heating the substrate 103 increases the power consumption, electric power saving particularly required in industrialization is not satisfied. Moreover, when a laser light source with a wavelength of, for example, 248 nm shorter than 300 nm is used (e.g., refer to Y. Taniguchi, et al., Extended Abstracts (The 51st Spring Meeting, 2004); The Japan Society of Applied Physics and Related Societies No. 2, P929, 28a-ZG-3), light absorption takes place in an optical system composed of a concave lens 106, a convex lens 107, a phase shifter 108, and a mirror. In the optical system, heat corresponding to the amount of light absorbed is generated. In the heat generation, the temperature rises with time, which results in the blurring of the focal point of the lens and the displacement of the crystallization position. The displacement leads to a shift in the crystallization region. When a transistor circuit is formed, the channel region is formed off the crystallization region in the exposure process, with the result that the yield becomes worse in the mass production process. Furthermore, in a display device, it is understood that display irregularity and color nonuniformity occur, which causes poor display.